Until now, known propulsion units of this type, whether they be purely outboard motors or so called inboard propulsion units with totally submerged or surface-breaking propellers, have had propeller shafts which, when the boat is travelling straight ahead, have been aligned with the longitudinal extension of the boat. In other words, the propeller shaft has always been parallel to the usual steer fin on outboard motors and inboard propulsion units.
When powered by surface-breaking propellers, water is accelerated both in the axial direction and sideways, whereby the sideways forces generated by the sideways acceleration in previously known propulsion units has solely resulted in pure losses of kinetic energy, which is comparable with the rotation of the slip stream in conventional propellers. The resultant of the propeller pressure force in the longitudinal direction and the sideways force can, when large sideways forces are present, deviate so much from the longitudinal direction that it has been necessary to compensate for this by setting the rudder of the vessel (with fixed drive units), or the entire propulsion unit (with simple steerable units), at an angle, with consequential increase in the flow resistance. In twin installations, the counter-rotating propellers do not require any oblique setting of the propulsion unit or rudder since the side forces act in opposite direction and do not generate any steering forces, though the losses remain and the side forces must be taken up in steering components, tie-rods and suspension members.